Modifiers matching automobile:

• private,
• bulky,
• rental,
• recreational,
• large,
• big,
• efficient,
• fighting,
• new,
• current,
• autonomous,
• inside,
• smaller,
• sized,

Simple categories matching efficient:

• equipment,
• performance,
• appliance,
• commodity,
• component,
• vehicle,
• process,
• ballast,
• organization,
• unit,
• device,
• energy,
• product,
• automobile,
• activity,
• fuel,
• service,
• facility,
• machine,
• fixture,
• container,
• ministry,
• engine,
• institution,
• technology,
• stove,
• meat,
• system,
• thing,
• practice,
• laser,
• business,
• ionization,
• measure,
• truck,
• gear,
• item,
• means,
• creature,
• network,
• action,
• manner,
• way,
• transport,
• utility,
• port,
• radiator,